1. Field of the Invention
The present invention relates to a VDSL-modem.
2. Discussion of the Background
One of possible technologies for FTTN (Fibre to the Neighbourhood) is VDSL (Very High Data Rate Digital Subscriber Line). In simple terms VDSL transfers high data rates over short distances in copper cable in the Public Switched Telephone Network with a rate which depends on the current length of the copper cable. The maximum downlink rate varies between 51 and 55 Mbit/s over copper cable with a lenght of up to 300 m. The downlink rate is about 13 Mbit/s over a copper cable with a length of about 1500 m. The uplink rate in the previous VDSL-models was asymmetric just as ADSL, but with a rate of about 1,6–2,3 Mbit/s. Both data channels are separated in frequency from the channel bands which are used in POTS and ISDN, which makes possible for service suppliers to superimpose VDSL on existing services. At present also the two high speed channels for uplink respective downlink are separated in frequency. When the need increases for higher rates in the uplink channel, or symmetric rates, the VDSL-systems need to use echo cancelling. In order to correct errors which occur due to noise, VDSL utilizes for instance Forward Error Correction (FEC) with sufficient interleaving for correction of all errors.
At use of VDSL-modems for providing broadband services the lenght of the last copper line up to the subscriber is a limiting factor. This distance must be made as short as possible. FIG. 1 shows how a VDSL-modem according to the prior art is arranged in a node between a subscriber and a local station (exchange; X). The distance from the node with the VDSL-modem to the subscriber is about 300 m, at which the transmission rate may be about 50 Mbit/s in the downlink. The transmission between the node and the exchange (X) is made by means of fibre. As has been mentioned above, it is important to keep the distance between the node and the subscriber as short as possible. One way of effecting this is to extend the network with special optical nodes (ONU), as can be seen in FIG. 1, between the local station (exchange) and the subscriber. The optical fibre network consequently shall reach all these nodes, and the location is selected with intention to minimize the lenghts of the copper line. According to FIG. 1 one so far has placed the whole VDSL-modem in these nodes and made these communicate with local station (the exchange).
One problem by arranging the whole VDSL-modem in the optical node is that one will have problems with the synchronization of different VDSL-modems. The synchronization between these modems is of greatest importance, for instance for the duplex method which is utilized in Telia's patent application 9502775-1, which by this is included by reference.
Another problem is that the multiplexor function in the optical node is complicated and must be able to handle different protocols such as ATM, IP etc.
The aim with the present invention consequently is to solve these problems and reduce the complexity in the optical nodes.